This Invention relates to the acquisition and display of a sequence of images, and particularly to video cameras and recorders used to document criminal activity and other events occurring in a monitored area.
Video surveillance cameras are required to operate under a wide range of lighting conditions, from natural sunlight to indoor and outdoor manmade illumination. A particularly common lighting technology is the gas discharge lamp, the most prevalent examples of which are fluorescent and sodium vapor lights. These devices operate by applying a large alternating voltage across a gas filled enclosure, usually with the same frequency as the power line, i.e., 50 or 60 Hz. One pulse of light is emitted from the enclosed gas on the positive excursion of the voltage, and another pulse on the negative excursion. Accordingly, the light emitted from gas discharge lamps pulsates or flickers at twice the line frequency, i.e., 100 or 120 Hz. This flicker rate is fast enough that it cannot be detected with the human eye, and therefore these lighting systems can be use in general applications, both indoor and outdoor.
Video surveillance cameras capture images far faster than the human eye, and are therefore susceptible to the degrading effects of lighting flicker. Conventional video surveillance cameras, using two-dimensional image sensors, overcome this problem by acquiring each image in phase synchronization with the power line frequency. In the most common case, this involves acquiring 50 fields/second at a power line frequency of 50 Hz, or 60 fields/second at 60 Hz. This results in each image in the video sequence being acquired under the same lighting conditions, thereby eliminating the effects of lighting flicker.
However, the problem of flicker is much more severe in video surveillance systems that utilize linescan image sensors, such as disclosed in U.S. Pat. No. 6,757,008, which is incorporated herein by reference. These systems operate by acquiring each image line by line, rather than an entire image at once. Accordingly, each line must be acquired at a rate that is far higher than the power line frequency, typically 5,000 to 30,000 lines/second. This results in each line being acquired under a different lighting condition, depending on the particular phase of the periodic illumination. The acquired image therefore shows pronounced bright and dark stripes that degrade the image's usefulness.